Synthesis of organic bisurea compounds and their roles as crystallization nucleating agents of poly(l-lactic acid)

A series of novel organic bisurea (BUr) compounds were facilely synthesized from phenyl isocyanate and a variety of diamines, and their roles as crystallization nucleating agents of commercial poly(l-lactic acid) (PLLA) were investigated through differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide-angle X-ray diffraction (WAXD). Their crystallization accelerating efficiency depends on the methylene numbers in molecular structure: the less the CH2 number, the faster the crystallization rate. Among them, 1,1′-(ethane-1,2-diyl)bis(3-phenylurea) (BUr1) acts as the most effective crystallization accelerating agent. In the presence of ca. 2 wt% BUr1, the melt crystallization enthalpy of PLLA increases from almost 0 to 31.2 J/g under cooling rate of 10 °C/min, and the isothermal melt crystallization rate (image) increased by 10.5 times at 140 °C. Much higher nucleation density and smaller spherulite size of PLLA were observed by POM, indicating obvious heterogeneous nucleation effect. And the crystal structure of PLLA remains unchanged after incorporation of the nucleator.